The poll, conducted by GOP pollster Basswood Research for the Club for Growth and Senate Conservatives Fund, shows Pryor's poll plunging following a series of ads from the groups tying him to President Obama. His favorable and unfavorable numbers are even at 36 percent, according to the poll, down from a positive 53 to 25 percent favorability split in January, before the groups went on the air.

The poll also shows Rep. Tom CottonThomas (Tom) Bryant CottonLawmakers introduce bill to block U.S. companies from doing business with Huawei Five things to know about Iran's breaches of the nuclear deal Hillicon Valley: Trump gets pushback after reversing course on Huawei | China installing surveillance apps on visitors' phones | Internet provider Cloudflare suffers outage | Consumer groups look to stop Facebook cryptocurrency MORE (R-Ark.) leading Pryor in a hypothetical matchup by 43 to 35 percent. Cotton, a Tea Party favorite well-liked by the groups, has refused to discuss whether he's considering running for the seat.

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Partisan polls should always be taken with a grain of salt — but that's a big dip for a sitting senator, and should concern Pryor and Senate Democrats. He's long been considered one of the most vulnerable incumbents.

The Democratic Senatorial Campaign Committee pushed back on the poll.

"The people of Arkansas elected Tom Cotton five minutes ago," said DSCC spokesman Matt Canter. "If he already deems himself worthy of a promotion, then he is in for a lot of disappointment. The fact is that this sham poll is brought to you by the Joe Miller, Richard Mourdock, Todd Akin wing of the Republican party. Reince Priebus tried to excommunicate these guys yesterday."

Both polls were of 600 registered voters and had margins of error of plus or minus 4 percentage points. The most recent poll was conducted from March 16-17, while the first poll was from Jan. 27-28.

This post was updated at 10:15 a.m.